The Mechanism of Chromatographic Retention - Solvent/Solute Interactions with Adsorbent Surfaces > Bi-layer Adsorption > Page 50

Figure 21. Individual and Combined Adsorption Isotherms for Ethyl Acetate on Silica Gel

The individual isotherms for the two adsorbed layers of ethyl acetate are included in figure (21). The two curves are of the same form but quite different in magnitude. The first layer is complete when the concentration of ethyl acetate in the mobile phase is only about 1%w/w. As the concentration of ethyl acetate rises above 1%w/w the second layer is only just started forming. The second layer of ethyl acetate forms much slower and obviously the interactions between the solvent molecules with those already adsorbed on the surface are much weaker than their interaction with the silica gel silanol groups.